Solid state photomultiplier

ABSTRACT

Embodiments of a solid state photomultiplier are provided herein. In some embodiments, a photosensor may include a sensing element; and readout electronics, wherein the sensing element is AC coupled to the readout electronics. In some embodiments, a solid state photomultipler may include a microcell having; a sensing element; and readout electronics, wherein the sensing element is AC coupled to the readout electronics.

CROSS REFERENCE TO RELATED APPLICATION

This patent application claims the benefit of priority, under 35 U.S.C.§119, of U.S. Provisional Patent Application Ser. No. 62/053487, filedSep. 22, 2014, titled “SOLID STATE PHOTOMULTIPLIER” the entiredisclosure of which is incorporated herein by reference.

BACKGROUND

The subject matter disclosed herein generally relates to detectors foruse in imaging systems, such as X-ray, nuclear medicine imaging systems,combinations thereof, or the like.

Conventional imagining technologies generally include one or moredetectors configured to convert incident radiation to useful electricalsignals that can be used in image formation. Such detectors may employsolid state photomultipliers (e.g., silicon photomultipliers (SiPM)),which may be useful for detecting optical signals generated in ascintillator in response to the incident radiation. Typical mechanismsutilized to read out analog SSPM pixels may include either AC or DCcoupling of the SSPM signal to external electronics. However, due tostray or parasitic capacitance along the signal path, the signal may bedegraded, thereby causing the detector to suffer from crosstalk, signalintegrity degradation and additional noise.

The inventors have observed that integrating the readout electronicswith the SSPM on the same die may be one mechanism to reduce suchcrosstalk or signal noise, and preserve signal integrity. Suchmechanisms typically including a photodiode (e.g., single-photonavalanche diode (SPAD) (fabricated in a high voltage well) on the samedie as the readout electronics (fabricated in a low voltage well) andinterfacing the SPAD and readout electronics to DC couple the signalgenerated by the SPAD to readout electronics. However, theseconfigurations require the die to be specifically fabricated tofacilitate isolation between the SPAD and the electronics. Moreover,such configurations may still suffer crosstalk between the high voltagecomponents (SPAD) and low voltage components (readout electronics).

Thus, the inventers have provided an improved solid statephotomultiplier.

BRIEF DESCRIPTION OF THE INVENTION

Embodiments of a solid state photomultiplier are provided herein. Insome embodiments, a photosensor may include a sensing element; andreadout electronics, wherein the sensing element is AC coupled to thereadout electronics.

In some embodiments, a solid state photomultipler may include amicrocell having; a sensing element; and readout electronics, whereinthe sensing element is AC coupled to the readout electronics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatical representation of an exemplary PET imagingsystem in accordance with some aspects of the present invention.

FIG. 2 is a block diagram of an exemplary conventional front-end readoutelectronics for a PET data acquisition system.

FIG. 3 depicts a perspective view of a detector element in accordancewith some aspects of the present invention.

FIG. 4 is a schematical view of an exemplary front-end readoutelectronics for a PET data acquisition system in accordance with someaspects of the present invention.

FIG. 5 is a block diagram of an exemplary microcell in accordance withsome aspects of the present invention.

FIG. 5A is a portion of the block diagram shown in FIG. 5 in accordancewith some aspects of the present invention.

FIG. 6 depicts a cross sectional view of a portion of a solid statephotomultiplier in accordance with some embodiments of the presentinvention.

FIG. 7 depicts a top down view of the portion of a solid statephotomultiplier shown in FIG. 6 in accordance with some embodiments ofthe present invention.

FIG. 8 depicts a cross sectional view of a portion of a solid statephotomultiplier in accordance with some embodiments of the presentinvention.

FIG. 9 depicts a top down view of the portion of a solid statephotomultiplier shown in FIG. 8 in accordance with some embodiments ofthe present invention.

FIG. 10 depicts a cross sectional view of a portion of a solid statephotomultiplier in accordance with some embodiments of the presentinvention.

FIG. 11 depicts a cross sectional view of a portion of a solid statephotomultiplier in accordance with some embodiments of the presentinvention.

FIG. 12 depicts a top down view of a portion of a solid statephotomultiplier in accordance with some embodiments of the presentinvention.

FIG. 13 depicts a top down view of a portion of a solid statephotomultiplier in accordance with some embodiments of the presentinvention.

DETAILED DESCRIPTION

Embodiments of a solid state photomultiplier are disclosed herein. In atleast some embodiments, the inventive solid state photomultiplieradvantageously utilizes a low voltage well that isolates low voltagereadout electronics (e.g. buffers, comparators, or the like.) from thehigh voltage components (e.g. photodiodes such as a single-photonavalanche diode (SPAD), or the like) to form a capacitor thatfacilitates an AC coupling of the signal generated by the high voltagecomponents to the readout electronics. Furthermore, this mechanism mayfurther advantageously be extended to allow the fabrication of on-chipcapacitors for AC coupling the high voltage signal to off-chip readoutelectronics.

Embodiments discussed herein relate to a detector in a nuclear imagingsystem, such as a positron emission tomography (PET) or single photonemission computed tomography (SPECT) imaging system or in a combined orhybrid imaging system including such PET or SPECT imaging functionality(e.g., a PET/MR, a PET/CT, or a SPECT/CT imaging system). It should beappreciated, however, that the present devices may also be employed inother types of imaging modalities or detectors used to detect radiationor nuclear particles, such as radiographic detectors used in X-ray basedimaging modalities (e.g., fluoroscopy, mammography, computed tomography(CT), tomosynthesis, angiography, and so forth). However, to simplifyexplanation, and to facilitate discussion in the context of a concreteexample, the present discussion will be provided in the context of anuclear imaging system.

FIG. 1 is a diagrammatical representation of an exemplary PET imagingsystem in accordance with some aspects of the present invention. Thougha PET system 110 is described and discussed herein, it should beappreciated that the present approach may also be useful in otherimaging contexts, such as in a SPECT or CT imaging system.

The depicted PET system 110 includes a detector assembly 112, dataacquisition circuitry 114, and image reconstruction and processingcircuitry 116. The detector assembly 112 of the PET system 110 typicallyincludes a number of detector modules (generally designated by referencenumeral 118) arranged about the imaging volume, as depicted in FIG. 1.As discussed herein the detector assembly 112, via the modules 118, maybe configured to generate signals in response to gamma rays generated bypositron annihilation events and emitted from a subject within theimaged volume. In certain implementations, the detector modules 118 caninclude scintillators and photon detection electronics. The detectorassembly 112 may be of any suitable construction and configuration foracquiring PET data. For example, as in the depicted example, thedetector assembly 112 can be configured as a full or partial ring.

In certain implementations, gamma rays may be converted, such as in ascintillator of the detector modules 118, to lower energy photons thatin turn may be detected and converted in the detector modules 118 toelectrical signals, which can be conditioned and processed to outputdigital signals. In certain imaging applications, to overcome the lownumber of optical photons generated in response to impinging radiationat the scintillator (i.e., the low signal level), a solid statephotomultiplier or silicon photomultiplier (SiPM) may be combined with ascintillator to provide amplification of the signals.

The signals generated by the detector modules 118 can be used to matchpairs of gamma ray detections as potential coincidence events. That is,in such a PET implementation, when two gamma rays strike opposingdetectors it may be determined that a positron annihilation occurredsomewhere on the line connecting the two impact locations (absent theeffects of interactions of randoms and scatter detections). In SPECTimplementations, line of flight information may instead be inferredbased at least in part on the collimation associated with the detectorassembly. The collected data can be sorted and integrated and used insubsequent processing such as by image reconstruction and processingcircuitry 116.

Thus, in operation, the detector acquisition circuitry 114 is used toread out the signals from the detector modules 118 of the detectorassembly 112, where the signals are generated in response to gamma raysemitted within the imaged volume. The signals acquired by the detectoracquisition circuitry 114 are provided to the image reconstruction andprocessing circuitry 116. The image reconstruction and processingcircuitry 116 generates an image based on the derived gamma ray emissionlocations. The operator workstation 126 is utilized by a system operatorto provide control instructions to some or all of the describedcomponents and for configuring the various operating parameters that aidin data acquisition and image generation. The operating workstation 126may also display the generated image. Alternatively, the generated imagemay be displayed at a remote viewing workstation, such as the imagedisplay workstation 128.

It should be appreciated that, to facilitate explanation and discussionof the operation of the PET system 110, the detector acquisitioncircuitry 114 and the image reconstruction and processing circuitry 116have been shown separately in FIG. 1 from other illustrated components(e.g., the detector assembly 112, the operator workstation 126, and theimage display workstation 128). However, it should be appreciated that,in certain implementations, some or all of these circuitries may beprovided as part of the detector assembly 112, the operator workstation126, and/or the image display workstation 128. For example, thehardware, software, and/or firmware executed on or provided as part ofthe data acquisition circuitry 114, whether provided as part of thedetector assembly 112, the operator workstation 126, and/or the imagedisplay workstation 128, may be used to perform various detector readoutand/or control actions described herein. In certain implementations thedata acquisition circuitry 114 may include specially configured orprogrammed hardware, memory, or processors (e.g., application-specificintegrated circuits (ASICs)) for performing detector readout steps asdiscussed herein. Similarly, certain of these readout functions may beperformed using one or more general or special purpose processors andstored code or algorithms configured to execute on such processors.Likewise, a combination of special purpose hardware and/or circuitry maybe used in conjunction with one or more processors configured to executestored code to implement the steps discussed herein.

With the preceding in mind, the detector technology in oneimplementation of a system such as that depicted in FIG. 1 will bediscussed in greater detail. In particular, a PET or SPECT system maycomprise a photosensor 120 that utilizes arrays of solid-state photomultiplier devices as part of the gamma ray detection mechanism, such aswithin detector modules 118. Solid state photomultipliers (SSPMs), whichare also commonly referred to as MicroPixel Photon Counters (MPPC) orMicroPixel Avalanche Photodiodes (MAPD) have become popular for use asphotosensors. Typically, SSPMs are implemented as SiliconPhotomultipliers (SiPM). Such devices may take the form, in certainimplementations, of an array of microcells (e.g., comprising passivelyquenched Geiger-mode avalanche photodiodes (APD)) for detectingimpinging photons. In general, SSPM devices used for photon detectioncan provide information about certain parameters, such as the time ofthe impingement event, the energy associated with the event, and theposition of the event within the detector. These parameters can bedetermined through processing algorithms applied to the output signalsgenerated by the SSPM.

In some embodiments, a multichannel readout front-endapplication-specific integrated circuit (ASIC) may interface with anarray of SSPMs in a PET (or SPECT) system. The ASIC may be provided aspart of the data acquisition circuitry 114 of FIG. 1 and may beconfigured to provide information on the timing, energy, and location ofevents in each SSPM to a processing system (e.g., processing circuitry116), as well as the ability to bias each SSPM.

Turning to FIG. 2, a block diagram is depicted representing one exampleof a front-end readout electronics of a PET data acquisition system 230,such as may be used with the PET system 110 of FIG. 1. The PET dataacquisition system 230 may include a plurality of pixels (SSPMs) 240 aswell as multiple ASICs 236 as part of the detector modules (118 ofFIG. 1) and/or data acquisition circuitry (114 of FIG. 1). Lightgenerated in a scintillator in response to a gamma ray interaction isdetected by a pixel and amplified. In this example, each SSPM 240includes an anode output 234 in electrical communication with the ASIC236 via a capacitor 238. That is, the outputs of the SSPMs 240 are theinputs to the respective ASIC 236. Each SSPM 240 may be furtherelectrically coupled to a resistor 242.

The ASIC 236, in turn provides one or more of timing signals, energysignals, and/or position signals as outputs. Each of these signalsoutput by the ASIC 236 corresponds to information obtained from therespective SSPMs 240 after processing by the ASIC 236. Although onlythree SSPMs 240 are shown in the figure, the PET data acquisition system230 may comprise any number of SSPMs 240 suitable to facilitate adesired functionality of the PET data acquisition system 230. Forexample, in some embodiments, the front-end readout electronics of adata acquisition system 230 may include eighteen (18) SSPMs 240.However, in other implementations, other quantities of SSPMs 240 may bepresent within a data acquisition system 230.

The solid state photomultipliers 240 may be fabricated using anymaterials suitable to provide the desired functionality as describedherein. For example, in some embodiments, each SSPM 240 may be formedusing silicon as a semiconductor material, although other suitablesemiconductor materials could instead be used (e.g. SiC, AlxGal-xAs,GaP, GaN and its alloys, amongst others).

In some embodiments, each SSPM 240 may include a plurality ofmicroscopic units, referred to as microcells. By way of illustration, asingle SSPM 240 is shown in FIG. 3 to illustrate certain of the presentconcepts. The number of microcells 346 on a SSPM 240 is typicallysufficient to provide effective dynamic range for the SSPM 240. The areaof a SSPM 240 is sufficient to cover one or more crystal elements 350formed on the scintillator 342. However, it should be appreciated thatthe exact number and density of the SSPMs 240 will be determined bydetector module design to achieve the optimal performance and otherknown factors.

As depicted in FIG. 3, a single SSPM 240 pixel is comprised of aplurality of microcells 346 that amplify single optical photon arrivalsfrom the scintillator 342 into an output signal, wherein each microcell346 comprises one or more APDs. Typically, each SSPM 240 will contain alarge number of microcells 346 (e.g., thereby providing between 100 to2,500 APDs per mm²) In some embodiments, each of the microcells 346 mayhave a length of between 20 microns to 100 microns. In oneimplementation, each of the microcells 346 may operate as an individualGeiger-mode APD a few volts above a breakdown voltage, with eachmicrocell 346 being virtually identical to all the other microcells. Inthis mode of operation, an electron or hole generated by the absorptionof an optical photon initiates an avalanche breakdown that is confinedto an individual microcell 346 when the one or more photons are absorbedby that microcell 346.

In some embodiments, each microcell 346 functions independently of theothers to detect photons. In such embodiments, a single discrete unit ofelectrical charge is emitted from the microcell 346 independent of thenumber of photons absorbed therein. That is, for each Geiger breakdown,the output signal of the microcell 346 will have substantially the sameshape and charge. In some embodiments, the microcells are electricallyconnected in parallel to yield an integrated current over some area overwhich the signals are being aggregated, such as a SSPM 240. The summeddischarge currents of the microcells 346 are indicative of the incidenceof radiation over a given area. This quasi-analog output is capable ofproviding magnitude information regarding the incident photon flux overthe area for which signals are being aggregated.

Conventional SSPM array configurations typically include coupling eachpixel (SSPM 240 of FIG. 2) to the ASIC/readout electronics (ASIC 236)via either AC or DC coupling of the SSPM signal to external electronics(shown schematically in FIG. 4). However, the inventors have observedthat due to a parasitic capacitance along the signal path, the signalmay be degraded, thereby causing the detector to suffer from cross talkand additional noise. Integrating readout electronics with the SSPM onthe same die may be one mechanism to reduce such crosstalk or signalnoise. Such mechanisms typically include interfacing a SPAD (fabricatedin high voltage well) on the same die to DC couple the signal generatedby SPAD to readout electronics (fabricated in low voltage well).However, these configurations require special isolation between the SPADand the electronics, and still suffer crosstalk between the high voltagecomponents and low voltage components.

As such, as discussed below, in some embodiments, the inventive solidstate photomultiplier advantageously utilizes an isolation well(described below to isolate the low voltage components (e.g. readoutelectronics, buffers, comparators, etc.) from the high voltagecomponents (e.g. SPAD (APD 502 discussed below)), and further, utilizesa capacitance formed by a structure of the isolation well to AC couplethe high voltage components to the low voltage components. Such ACcoupling may, for example, advantageously allow for a propagation ofsignal generated by the high voltage components (SPAD) to the lowvoltage components (readout electronics) while reducing or eliminatingthe increased noise, cross talk, or signal degradation discussed above.

One example of the above discussed capacitance is schematically shown inFIG. 5. In the depicted embodiment, the sensing element 512 of themicrocell 500 comprises an avalanche photodiode (APD) 502 and at leastone of impedance circuitry (e.g., a frequency dependent input impedancecircuit) 506 and a resistor 504 coupled thereto. In such embodiments,the APD 502 is coupled to the readout electronics 510 via a capacitor508 (e.g., the capacitor formed by the isolation well as describedherein). The impedance circuitry 506 may include any passive or activecomponents known in the art, for example, such as one or more resistors.Although shown as only having one APD 502, the microcell may be anynumber of APDs 502 suitable to provide a desired functionality of themicrocell 500. For example, in some embodiments, the microcell 500 mayinclude two or more, or an array, of APDs 502, such as the two APDs 502shown in FIG. 5A. In such embodiments, each APD 502 may be respectivelycoupled to two or more resistors (one resistor 504 (quench resistor)coupled to each APD 502 shown). As shown in the figure, when present,the two or more resistors 504 may each be coupled to the APDs 502 at afirst end 514 and to one another at a second end 516. In addition, thetwo or more resistors 504 may be further coupled to the readoutelectronics via the capacitor 508 and the impedance circuitry 506.

In some embodiments, the capacitance provided by the capacitor 508 maybe obtained via relative placement of high voltage and low voltageelements (e.g., CMOS wells) on a single wafer during fabrication of thesolid-state photo multiplier (e.g., SiPM). For example, referring to thecross sectional view of a portion of a solid state photomultiplier(SSPM) 600 in FIG. 6 and the top view of the portion of SSPM 600 in FIG.7, in some embodiments, the SSPM 600 may comprise a substrate 602 and afirst well (e.g., a high voltage well) 604 and second well (e.g., a lowvoltage well) 606 formed in the substrate 602. In such embodiments, thefirst well 604 may be coupled to the high voltage components of the SSPM600 (e.g., APD or SPAD) and the second well 606 may be coupled to thelow voltage components of the SSPM 600 (e.g., readout electronics). Thesubstrate 602 may be any type of substrate suitable for the fabricationof a SSPM 600, for example, such as a silicon based substrate, or thelike. In addition, the substrate 602 may be doped to form a p-type orn-type material (p-type shown in FIGS. 6 and 7).

The second well 606 may be doped to form either a p-type or n-type well(n-type well shown in FIGS. 6 and 7). In some embodiments, one or morenested wells (one p-type nested well 612 and one n-type nested well 614)may be formed within the second well 606.

In some embodiments, an isolation well 608 may be disposed between thefirst well 604 and second well 606. In such embodiments, the isolationwell 608 may be doped to form either a p-type or n-type well (n-typewell shown in FIGS. 6 and 7). Formed within the isolation well 608 areone or more nested wells having a type opposite the type of theisolation well 608, for example, such as the p-type nested well 610shown in FIGS. 6 and 7. The inventors have observed that the isolationwell 608 and nested well 610 structure provides the capacitance to ACcouple the high voltage components in the high voltage well 604 to thelow voltage components (in the low voltage well 606) as described above.

Although the substrate 602, first well 604, second well 606 andisolation well 608 and respective nested wells are shown in FIGS. 6 and7 as being of a certain type (e.g., p-type or n-type), it is to beunderstood that the wells may be of any type suitable to facilitate theoperation of the SSPM 600 as described herein. For example, the crosssectional view of a portion of a solid state photomultiplier (SSPM) 600in FIG. 8 and the top view of the portion of SSPM 600 in FIG. 9 depicteach of the components of the SSPM 600 having an opposing type ascompared to FIGS. 6 and 7.

While one configuration of the solid state photomultiplier 600 is shownin FIG. 6, it is to be understood that any configuration/placement ofthe high voltage well 604 relative to the low voltage well 606 suitableto provide the above described capacitance/coupling may be utilized. Forexample, in some embodiments, the low voltage well 606 may be disposedsufficiently close (e.g., as opposed to nested within, as shown in FIG.6) to the high voltage well 604 to provide the desiredcapacitance/coupling. In such embodiments, the orientation or placementof each of the high voltage well 604 and low voltage well 606 may beoptimized to offset a fringe electric field created by high voltagecomponents (e.g., APD or SPAD) that would otherwise have an effect onother components (e.g., low voltage elements). Such a fringe electricfield, or leakage effect thereof, may be determined utilizing one ormore algorithms or software tools conventionally utilized for highvoltage component design.

Although certain configurations of each of the wells to provide thedesired capacitance and facilitate the AC coupling of the low voltageand high voltage components are shown, the capacitance may be acquiredvia any suitable means known in the art. For example, FIG. 10 shows ageneral depiction of the coupling of the high voltage well 606 and thelow voltage well 604 via the isolation well 608 via a capacitor (shownin phantom at 616). Such coupling may be facilitated via any othersuitable type of capacitor fabricated in the isolation well, for examplesuch as a MOSFET gate capacitor, a conductor-insulator-conductorcapacitor, or the like.

Although shown in certain configurations above, the low voltage and highvoltage wells may be arranged in any manner suitable to provide the ACcoupling as described herein. For example, an exemplary configuration ofa portion of a microcell 1102 is depicted in FIGS. 11-12, where a lowvoltage well 1106 (e.g., low voltage well 606 described above) is shownnested within the high voltage well 1104 (e.g., high voltage well 604described above). Referring to the top down views in FIGS. 12 and 13, insome embodiments, at least a portion of the low voltage well 1106 may bedisposed about a periphery of the high voltage well 1104 and the APD1202. In such embodiments, the outer periphery of the microcell 1102 maybe configured to accommodate the readout electronics 1302, such as shownin FIG. 13. In some embodiments, multiple microcells (one additionalmicrocell shown in phantom at 1202) may be disposed adjacent to themicrocell 1102 to form an array 1206.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

1. A microcell for a photosensor, comprising: a sensing element; andreadout electronics, wherein the sensing element is AC coupled to thereadout electronics.
 2. The microcell of claim 1, further comprising acapacitor AC coupling the sensing element to the readout electronics,wherein the capacitor is formed by at least one low voltage well and atleast one high voltage well, wherein the at least one low voltage welland the at least one high voltage well are formed within a substrate. 3.The microcell of claim 2, further comprising: a plurality ofphotodiodes; a plurality of quenching resistors each having a first endrespectively coupled to the plurality of photodiodes and each having asecond end electrically coupled to one another and electrically coupledto the readout electronics via the capacitor.
 4. The microcell of claim2, wherein the low voltage well is disposed within the high voltagewell.
 5. The microcell of claim 2, wherein at least a portion of the lowvoltage well is disposed about a periphery of the high voltage well. 6.The microcell of claim 2, wherein the capacitor is formed by anisolation well disposed between the at least one high voltage well andthe at least one low voltage well.
 7. The microcell of claim 6, whereinthe isolation well comprises a nested well disposed within the isolationwell, wherein the isolation well comprises one of a p-type dopant or ann-type dopant and wherein the nested well comprises one of a p-typedopant or an n-type dopant that is opposite that of the isolation well.8. The microcell of claim 1, wherein the sensing element comprises anavalanche photodiode operating in Geiger mode above breakdown voltage.9. The microcell of claim 1, wherein the sensing element comprises atleast one of quench resistor and impedance circuitry.
 10. The microcellof claim 9, wherein the impedance circuitry comprises at least one ofpassive and active elements.
 11. A solid state photomultipler,comprising: a microcell having; a sensing element; and readoutelectronics, wherein the sensing element is AC coupled to the readoutelectronics.
 12. The solid state photomultipler of claim 11, furthercomprising a capacitor AC coupling the sensing element to the readoutelectronics, wherein the capacitor is formed by at least one low voltagewell and at least one high voltage well, wherein the at least one lowvoltage well and the at least one high voltage well are formed within asubstrate.
 13. The solid state photomultipler of claim 12, wherein themicrocell further comprises: a plurality of photodiodes; a plurality ofquenching resistors each having a first end respectively coupled to theplurality of photodiodes and each having a second end electricallycoupled to one another and electrically coupled to the readoutelectronics via the capacitor.
 14. The solid state photomultipler ofclaim 12, wherein the low voltage well is disposed within the highvoltage well.
 15. The solid state photomultipler of claim 12, wherein atleast a portion of the low voltage well is disposed about a periphery ofthe high voltage well.
 16. The solid state photomultipler of claim 12,wherein the capacitor is formed by an isolation well disposed betweenthe at least one high voltage well and the at least one low voltagewell.
 17. The solid state photomultipler of claim 16, wherein theisolation well comprises a nested well disposed within the isolationwell, wherein the isolation well comprises one of a p-type dopant or ann-type dopant and wherein the nested well comprises one of a p-typedopant or an n-type dopant that is opposite that of the isolation well.18. The solid state photomultipler of claim 11, wherein the photodiodecomprises an avalanche photodiode operating in Geiger mode abovebreakdown voltage.
 19. The solid state photomultipler of claim 11,wherein the sensing element comprises at least one of quench resistorand impedance circuitry.
 20. The solid state photomultipler of claim 19,wherein the impedance circuitry comprises at least one of passive andactive elements.